Project Summary/Abstract Recent years witnessed a worldwide reemergence of Pertussis (whooping cough, PT) despite widespread vaccination. In the mid-1990s, concerns over vaccine-related side effects prompted the widespread replacement of the whole-cell Pertussis (wP) vaccine in favor of a safer acellular Pertussis (aP) vaccine. Unexpectedly, whooping cough cases recently increased, particularly in teenagers. As this age bracket corresponds to the first cohorts of children that were aP-vaccinated in the mid-1990s as newborns, it is suspected that the switch to the aP vaccine may underlie this rise in morbidity, which is further supported by epidemiologic evidence comparing introduction of the aP vaccine and resurgence of PT cases across different countries. The goal of this proposal is identifying the mechanisms and correlates of long-lasting vaccine- induced immunity. Our approach is to compare individuals born before 1995 that were vaccinated in infancy with wP, with individuals born in 1996 or later that were vaccinated with aP in infancy. These young adults correspond to the population and age group in which the increased disease incidence is noted. Remarkably, we detect drastic qualitative differences in the immune phenotypes of the two populations despite the fact that the original aP- or wP-priming occurred more than 18 years ago. Ex vivo analysis of PT-specific T cells revealed that original aP priming is mostly associated with IL-4, TGF-? and IL-9 responses while original wP priming is associated with IFNg and IL-17 responses. Furthermore, after repeated aP vaccination boosts, T cells originally primed with aP become associated with diminished capacity to respond to a boost in vivo and have altered proliferative capacity. Here, we plan to define the transcriptional response to vaccine boost in PBMC from donors originally primed with aP vs. wP and identify the cell types associated with differential responses (Aim 1). Following this initial ?broad net? characterization, we will hone in the characterization of differences in PT-specific memory T cell responses in aP vs. wP donors (Aim2) and characterize the molecular mechanisms of differences in APC-priming of T cells in aP- vs. wP-vaccinated donors (Aim 3). Our study will be the first to characterize the interplay of APC and T cell response, comparing the outcome of priming with two vaccines associated with differential protective efficacy. The results will have implications for specifically understanding the immunological mechanisms associated with Pertussis vaccination and more generally defining the mechanisms of durable vaccine efficacy.